Organic electroluminescent devices are a self-emitting device that provides better brightness and visibility than liquid crystal devices, and thus realize clearer display. For this reason, there have been active studies on organic electroluminescent devices.
In an attempt to improve the luminous efficiency of the devices, devices that phosphoresce with the use of phosphorescent material, specifically that utilize the emission from the triplet excited state have been developed. According to the theory of excitation state, the use of phosphorescent emission realizes luminous efficiency as high as about 4 times that of conventional fluorescence, and a prominent increase can be expected for luminous efficiency.
In 1993, M. A. Baldo and others of Princeton University achieved an 8% external quantum efficiency with a phosphorescent device using an iridium complex.
Because phosphorescent material undergoes concentration quenching, the phosphorescent material is held by being doped in charge-transporting compounds, or host compounds as they are generally called. The phosphorescent material held in this manner is called a guest compound. The 4,4′-di(N-carbazolyl)biphenyl (hereinafter, “CBP”) of the following formula has been commonly used as such host compounds (see, for example, Non-Patent Document 1).

However, because CBP is highly crystalline, the poor stability in the thin-film state has been pointed out. Accordingly, it has not been possible to obtain device characteristics, such as high-luminance emission, that are satisfactory in situations requiring heat resistance.
In this connection, the 4,4′,4″-tri(N-carbazolyl)triphenylamine (hereinafter, “TCTA”) of the following formula has been proposed as a new host compound, and luminous efficiency comparable to that of CBP has been confirmed (see, for example, Non-Patent Document 2).

As the studies of phosphorescent device progress, there is an increased level of understanding of the energy transfer process between the phosphorescent material and the host compound. This has made it clear that the excited triplet levels of the host compound need to be higher than the excited triplet levels of the phosphorescent material for improved luminous efficiency. Accordingly, there is a need for a host compound having higher excited triplet levels than CBP. Studies of host compounds having higher excited triplet levels have found that doping an electron-transporting or bipolar-transporting host compound with an iridium complex can yield higher luminous efficiency (see, for example, Non-Patent Document 3).
Further, there is a light emitting layer produced by doping the green phosphorescent material Ir(ppy)3 of the following formula
in a mixed host compound that includes the hole-transporting host compound TCTA and the electron-transporting host compound TPBI of the following formula.

The use of TCTA for the triplet exciton-confining electron blocking layer can achieve high efficiency and low voltage driving (see, for example, Non-Patent Document 4).
On the other hand, the external quantum efficiency of a phosphorescent device in which the CBP doped with the blue phosphorescent material FIrpic of the following formula is used as the host compound of the light emitting layer remains only at 6%.

This is considered to be due to the insufficient confinement of the triplet excitons by FIrpic, attributed to the lower excited triplet level 2.56 eV of CBP compared to the excited triplet level 2.62 eV of FIrpic.
This has been demonstrated by the temperature-dependent photoluminescence intensity of a thin film produced by doping CBP with FIrpic (see, for example, Non-Patent Document 5).
Further, the excited triplet level of the TCTA used as the electron blocking layer of the green phosphorescent device is 2.60 eV, a value still considered to be insufficient for the confinement of the FIrpic triplet excitons.
Accordingly, there is a need for a light emitting layer host compound and an electron-blocking compound that can completely confine the triplet excitons of the phosphorescent material, in order to improve the luminous efficiency of the phosphorescent device.